1. Field of the Invention
This invention pertains to cathode ray tubes (CRTs) of the type having an electron lens for amplifying the deflections of their electron beam, in combination with a postaccelerating or intensifier electrode. More specifically the invention concerns an improved electron lens for deflection amplification, herein referred to as a scan expansion lens, for particular use in oscilloscope and storage CRTs.
2. Description of the Prior Art
Postacceleration or postdeflection acceleration CRTs of usual prior art construction increase the velocity of their beam electrons, after they have traversed the deflection fields, by virtue of the accelerating field created by a planar or curved mesh and a postaccelerating electrode on the inside surface of the bulb or envelope. Thus postaccelerated, the beam provides a spot of increased brilliance on the fluorescent screen.
The mesh incorporated in this known type of CRTs, however causes a decrease in their electron gun efficiency, a defocusing of the beam spot on the screen, and halation due to secondary emission from the mesh. Recent effort in the electronics industry has therefore been directed toward the development of meshless postacceleration CRTs.
U.S. Pat. No. 3,496,406 to Deschamps describes and claims one such meshless postacceleration CRT, which comprises a quadrupolar electron lens for deflection amplification and a dome-shaped, apertured electrode. While capable of eliminating the drawbacks of the meshes to a certain extent, this postacceleration device possesses its own disadvantages such as the defocusing or nonuniformity of the spot upon increase in deflection angles. The device also requires the use of only the central portion of the electron beam to provide a spot of favorable characteristics, so that its electron gun efficiency is lower than that of the mesh-type postacceleration CRTs. The device also gives rise to pattern distortions, which can be eliminated only by two additional electrodes provided exclusively for that purpose.
Another example of meshless postacceleration CRT is disclosed in the article entitled, "`Box` Lens Design Being Tried by Tektronix in Experimental CRTs," on page 50 of the May 26, 1977, issue of Electronics. The article deals with a box-shaped, four-electrode scan expansion lens in which the four lens elements are shaped in alternating convergent and divergent geometries and arranged serially with insulating gaps therebetween. The box-shaped scan expansion lens defeats many of the limitations of the meshes. For truly satisfactory display characteristics, however, the lens must measure 2.5 by 4.2 by 1 inch to provide an 8-by-10-centimeter display. This makes the size of the CRT rather inordinately large.